Are cheap UPS any good against power dips and brownouts?

I have several UPS for my computers. All of them are cheap POS, save for a high quality one made by APC.

This APC UPS has several LEDS on the front panel indicating among other things undervoltage, overvoltage and load level. Occasionally it will emmit a single beep and the undervoltage LED will turn on for a while. The computers are in a semi-industrial environment with some heavy machinery and we do encounter some power sags from time to time.

But how about the cheap UPS? They offer no indication that there is anything wrong going on. Do they offer any protection against sags?

The cheap ones will protect against sags, as long as the voltage goes below whatever the unit’s threshold for activation is - probably in the area of 85-90 volts.

If you want one that regulates the voltage to 120 +/- some amount, look for AVR or automatic voltage regulation, or some mention of buck/boost.

For my money, APC is the brand to get - an APC unit alerted me to a “loose neutral” a few years back. It started putting out an alarm that the incoming voltage was over 140 volts, and I was able to have the power utility make the needed repairs before anything in the building was damaged.

“Cheap” is a bit subjective, but I can recommend this line of CyberPower UPSes:

They isolate your device entirely from the wall power, feeding it “perfect” (nearly perfect) 120v sinewave no matter what’s going on with the rest of the wiring. Price-wise, they’re not bottom-of-the-barrel but I think they’re quite reasonable.

I would second this suggestion. I’ve used quite a few CyberPower units both at home and in the workplace, including some heavy-duty rackmount units for servers. My experience with them has been very positive, more so than with APC units. I’m not sure why, but the APCs we used to use tended to go through their batteries very quickly, while the CyberPowers held out much longer. In fact, I’d estimate somewhere between 3 and 4 times longer battery life from the CyberPower units.

I also use several smaller CyberPower units much like the ones in the link (just and older model) in my home office. I had a loose neutral incident as well recently and my UPSs performed quite well. I would normally simply power-down when something like that happens, but there was something of a work emergency where was was at stake was worth much more then my computer (since the data was backed up safely. . .). My voltage was swinging between 70 and 140 volts and my computer operated totally normally for a couple of hours.

Ye, they do offer protection against power dips, sags, & brownouts.
What they do NOT protect against are power spikes (transients & overvoltages).

And worse, the cheapest designs offer only one-time protection against power spikes – the first power spike burns out the component that handles spikes, so that future spikes will pass thru to your machine uncontrolled. But nothing changes in the way it operates, so nothing indicates to you that you no longer have any protection against power spikes.

For me:
At the wall I start with a designated computer circuit.
Then a sacrificial surge protector (cheap) that I expect to burn out with each surge.
Then a good gang plug like a ‘TripLite’. It also has another surge protector in that. Then to a UPS back up that uses the battery to make the 120V so that there is no blip of any kind from power fluctuation.

I also run two auto batteries in parallel so I have enough battery for the system being run, to go for 3-5 hours & I can actually boot up the system from cold. Used to live with really bad electrical supply out in the woods on a mountain.

I buy back-ups at yard sales, usually just need batteries so I have cheap back-up for land line phone systems, modem & router on their own dedicated ups, and the TV and associated units.

I have several back-up units all ready to go and am running only three units in my present configuration because my Linux & 2 XP computers are not online at this time. ( wife has her own W-7 & XP computers, we don’t like to share machines. he he he )

Also have a voltage smoother that I used for the bad electric on the mountain but it is also waiting because we moved to an area with better & more consistent service.

I never have too many back ups.

Much cheaper than buying the computer bits needed from bad electric or lightning strikes.

Still unplug TV & puters etc. and just turn off a couple of things when a bad tornado warning or big & often lighting strikes are in the area. We had a big hot strike about 25 feet away from the house on the mountain during one bad storm and the EMP was so bad that it got several things still plugged in like clocks and stuff and one small unit that was not even plugged in. Scared the **** out of us.


I was once given a huge, massive, heavy-as-hell isolation transformer. The idea is that the transformer will work to smooth out most commonplace electric-company power flaws, and will also serve as some small protection against a lightning bolt.

Is the damn thing worth a pip, or should I just hurl it and be glad to be rid of it?

I think the APC units come in two flavours: Smart-UPS and Back-UPS. The “back” units only protect against power failure and give a square wave output. The “smart” units do brown out protection and provide a sine wave output, but are correspondingly more expensive.

These were all the rage way back in the 80s and early 90s, when UPSs weren’t available (or were prohibitively expensive).

They do kinda work. The transformer is essentially a big-ass inductor and as such does some filtering. It can store some energy so it can take care of the occasional power dip, as long as it is short in duration.

Problem is that nowadays power supplies use earth for filtering purposes. If you isolate your computer from earth (as these transformers do) you might run into several problems. I was too given one of these monsters and tried it on my computer about 10-15 years ago. I had lots of noise from the sound card and feedback in the mic. Also my CRT monitor would go funky and needed regular degaussing.

it is still useful for many purposes in electronics. very useful for safety when testing and servicing. they do use up power to run.

not as useful with computers with a UPS that does power conditioning besides the standby power.

if you don’t need it then look for someone who can use it.

I use a UPS for my C-pap machine, but it’s horrible. When the power cuts out and the battery power engages, the UPS starts beeping, which wakes me up, which defeats the entire purpose. Does anyone know of a UPS that stays shtum?

Both of my APC UPS allow me to turn off the beeping.

There are many UPS devices that can be programed to not alarm when on UPS power. Some require you to hook it up to a PC, others have a button of some sort.

Open it up & cut the speaker off with wire cutters.

Power off & unplugged while you do it of course.

a) a computer must work just fine even when incandescent bulbs dim to less than 40% intensity. How often does your voltage drop that low?

b) a UPS, not located at the service entrance, is often made as cheap as possible. It can become confused even by noise and other lesser anomalies. Then declare a low voltage when none really exists.

Its purpose is to switch to batteries when voltage drops below a fixed number (ie 104 volts). So that any deep brownout or blackout does not cause a loss of unsaved data. That occurs rarely. But a UPS may become confused and switch to batteries frequently when no voltage sag exists,

Again, watch a light bulb. How often does it dim?

c) a transformer also does not do what you have assumed. First, all computers must have galvanic isolation - a transformer. Second, computers are required to work for a defined period even when no AC power is incoming. Essential because a UPS has a period of no power as it switches from AC to batteries. As the UPS is providing no power, reserves inside a computer’s power supply provide rock solid and stable DC voltages.

A UPS has one purpose. To provide temporary and dirty power during a blackout. So that unsaved data can be saved.

d) Does not matter how ‘clean’ AC power is. Because the computer turns the cleanest power into some of the dirtiest higher voltages in the house. And then superior ‘cleaning’ filters convert the dirtiest power to rock solid, stable, and clean DC voltages.

But again, a computer must work even when an incandescent bulb dims to 40% intensity. The UPS provides temporary power if voltage drops lower - a bulb dims to below 40%. If your power repeatedly dims that much, then electric service needs immediate attention. Frequent dimming might even indicate a serious human safety threat.

Why do you always pop up Tom when anyone is discussing UPS?

So what if its durty power, its not like the UPS is on battery mode 24/7 is it ffs.

A UPS is great if your away from your pc and a storm hits, piece of mind. Yeah Id be itching to unconnected my pc in a storm as I wouldnt and dont fully trust my UPS in a storm, but if your out you cant really do much about it and I’ll tell you this, I would much perfer it connected to a UPS then to have no protection at all when a storm hits… Yeah a direct lightning hit would kill anything, but how rarely do you ever get a direct hit, a thunder storm is normally a few surges and power going up,down, off, on and Im sure a decent UPS with AVR could deal with that easy.

Cheap UPS systems tend to perform badly in two specific areas.

First, they don’t output a “clean” sine wave. Instead they often put out “stepped” waves that only approximate a sine wave. So instead of a nice sine wave you may end up with something like this:

All of those square steps can be a bit rough on the computer’s power supply.

Second, they provide very little protection against things like power spikes and noise. Brownouts are often accompanied by spikes and erratic voltages when things are switched back on and some of these will go right through a cheap UPS.

The performance of a cheap UPS can also be fairly poor in terms of how much current they can supply and how long their batteries last.

I have no idea where you got your 40% number from but the ATX power supply spec specifies 90 to 135 volts for proper operation. While there is nothing preventing power supply manufacturers from exceeding those specs, most computers are going to stop working long before light bulbs dim to 40%.

The actual spec is here if you want to read it (warning - PDF):

Dirty power, by the way, is power that is sagging or contains spikes or other forms of noise. Clean power is within spec, noise free, and is pretty close to a pure sine wave.

40% intensity for an incandescent light bulb is not the same thing as 40% voltage; only something like a single LED with a series resistor would respond linearly to a variation in line voltage (also one reason why dimmable CFL/LED lights don’t work as well). According to this calculator (enter some numbers for it to work), 40% intensity corresponds to about 76-77% nominal voltage, or 91-92 vac for a 120v nominal input - which is within the 90-135 vac range you specify (of course, many are now universal input, 90-270 vac or so, but a switched supply would be 90-135 or 180-270 vac).

Formulas are published even in the IES Handbook - the industry standard for light bulbs. We are saying similar using different standards. Most layman do not monitor voltage with a meter. Most layman can see how often voltage drops below 100 volts. Simply watch a light bulb change intensity. Almost never. At 40% intensity - less often.

A 40% intensity number is accurate (as Michael63129 notes. AVR is also already inside electronics per his numbers). You assumed a linear relationship. Intel’s ATX standards (and your citation) define a normal computer voltage. A bulb at 40% intensity is same. Every consumer can watch light bulbs to learn how often a UPS is really needed.

My 120 volt UPS claims to output pure sine waves. In battery backup mode, it outputs 200 volt square waves with a spike of up to 270 volts. From high school math, that is a sum of pure sine waves. They did not put a number to ‘pure’. So they did not lie. It outputs pure sine waves. And is sufficient power for any electronics.

What some call a surge is really only noise. Otherwise a typical UPS is outputting surges. Electronics already make such anomalies irrelevant. Electronics even contain an AVR. Why would anyone need a more expensive UPS with AVR?

A UPS (expensive or cheap) has one purpose. Temporary power so that unsaved data can be saved. An expensive one may include exotic front panel lights or a signal to power off the adjacent computer. Both perform the same primary function. Temporary power when incandescent bulbs dim to less than 40% intensity.

I actually assumed that the light output would be linear with respect to the power which would put the 40% point down around 70 volts or so (then again it’s late and I could have whiffed the math). Either way it doesn’t seem to have been correct so thanks for the correction.